There is a need for efficient storage of thermal energy within the area of modern energy technology.
Thermal energy may advantageously be stored in a fluid, such as e.g. water, above ground in insulated tanks, in ground in insulated pits, or underground in excavated caverns, using the surrounding ground as insulation. The thermal energy of the fluid is preserved to a great extent during an extended period of time. Today, these methods are used in different parts of the world in order to satisfy the need for storing thermal energy between different seasons, e.g. storing temporary surplus heat which is used later on when there is a demand for it and, preferably, when its financial value is higher. The main transition of energy is from the summer half, when there is less need for heating, to the winter half, when the need for heating is much higher. However, there is also much to gain by using the storage for short-term variations and always actively storing surplus heat. These kinds of storages may also be used for storage of a colder fluid, to be used for cooling, as well as for fluid having an intermediate temperature, such as a fluid used in low temperature systems.
When storing thermal energy underground, one must consider that the warmer the fluid becomes, the more it expands, and hence the more space the fluid requires. Further, one must consider the possibility of leakage or accidents where fluid enters into areas of the storage where it is not supposed to be, and hence the easy, fast and safe removal of such fluid. Further, one must consider the influence which the storage might have on the surrounding ground water level.
Swedish patent application 0950576-9 discloses one kind of efficient storage of thermal energy. However, there is still a need for an even more improved arrangement for storing thermal energy underground.